This invention relates to liquid metal cooled nuclear reactors.
A liquid metal cooled nuclear reactor is usually housed in a steel vessel which is itself enclosed within a steel lined concrete containment vault and in order to prevent damage to the concrete due to heat radiated from the nuclear reactor it is usual to interpose thermally insulating material between the reactor containing vessel and the vault lining. In one typical nuclear reactor construction known as a liquid metal cooled fast breeder nuclear reactor a fuel assembly is submerged in a pool of liquid sodium contained in a closed primary vessel and the primary vessel is suspended at its rim from the roof of a concrete containment vault having a membrane lining. The normal operating temperature of the primary vessel is approximately 400.degree. C. so that thermally insulating material is required to reduce heat radiation to the concrete. In the event of an emergency such as a breach of the primary vessel resulting in loss of sodium from the pool the nuclear reactor is automatically shut down but nevertheless circumstances are envisaged wherein, due to decay heat generated in the fuel, the temperature of the sodium could rise to an excessively high value (approximately 650.degree. C.) unless the excess heat is dissipated through the structure. Thus we have two conflicting requirements; thermally insulating material is required to reduce heat transfer to the concrete during normal operation of the reactor but, in the event of a breach of the primary vessel resulting in loss of coolant from the fuel assembly it must at least afford reduced heat insulation and preferably enhance heat transfer to the vault structure.
It is an object of the invention to provide a construction of liquid metal cooled nuclear reactor wherein a concrete containment vault is lined with thermal insulation to protect the vault against heat radiated from the reactor during normal operation of the reactor but wherein the efficiency of the heat insulation is reduced in an emergency to enable excessive heat from the reactor to be dissipated through the vault.